The clade Kinosternoidea consists of the extant mud and musk turtles (Kinosternidae) and the Central American river turtle Dermatemys mawii. Baptemys, an Eocene turtle taxon from North America, has historically been allied to D. mawii within Dermatemydidae, but this relationship has never been rigorously tested in a global analysis. Molecular data and multiple morphological characters support monophyly of Kinosternoidea, but kinosternids and D. mawii are vastly different in their morphology, and the relationships of Dermatemys are controversial. Dermatemys mawii is highly adapted to consuming aquatic vegetation and is thus much more similar in gestalt to some emydids than to kinosternids. Dermatemys mawii was historically placed among tortoises (Testudinoidea) by a number of traits pertaining to their fully ossified shell and the development of a secondary palate. Different placements of D. mawii indicate radically different historical biogeographic scenarios and sequences of character evolution. Few relevant morphological characters have been used in global analyses of turtle relationships, and several fossil taxa are known that could prove critical to resolving this debate. Baptemys wyomingensis is the best-sampled fossil dermatemydid. A detailed description of B. wyomingensis, along with a consideration of its phylogenetic relationships, indicates additional morphological support for a close relationship with Dermatemys and a placement for D. mawii and Baptemys within Kinosternoidea, as well as an unexpected close relationship with Hoplochelys and Agomphus to the exclusion of the Kinosternids. A review of the alpha taxonomy of Baptemys reveals that the relationships between the species, other than B. wyomingensis and B. garmanii remain unclear due to a lack of published descriptions and it appears likely that Baptemys may be paraphyletic in regard to D. mawii.